U.S. patent number 5,746,214 [Application Number 08/416,717] was granted by the patent office on 1998-05-05 for investigation of a body.
This patent grant is currently assigned to British Technology Group Limited. Invention is credited to David Charles Barber, Brian Hilton Brown.
United States Patent |
5,746,214 |
Brown , et al. |
May 5, 1998 |
Investigation of a body
Abstract
A method of investigation of a body including applying
interrogatory electrical signals at different frequencies to the
body. First signals are obtained representing first electrical
impedance measurements at the different frequencies. Second signals
are obtained representing subsequent second electrical impedance
measurements at the different frequencies after a change in the
internal state of the body. Characteristics of part of the body are
selectively determined in response to the first and second signals
at the different frequencies. The invention has application in
electrical impedance tomography techniques where the different
frequency behavior of different parts of the body associated with
temporal changes can be used to improve organ resolution and tissue
differentiation.
Inventors: |
Brown; Brian Hilton (Sheffield,
GB2), Barber; David Charles (Sheffield,
GB2) |
Assignee: |
British Technology Group
Limited (London, GB2)
|
Family
ID: |
10724375 |
Appl.
No.: |
08/416,717 |
Filed: |
May 30, 1995 |
PCT
Filed: |
October 28, 1993 |
PCT No.: |
PCT/GB93/02223 |
371
Date: |
May 30, 1995 |
102(e)
Date: |
May 30, 1995 |
PCT
Pub. No.: |
WO94/09699 |
PCT
Pub. Date: |
May 11, 1994 |
Foreign Application Priority Data
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|
|
|
Oct 30, 1992 [GB] |
|
|
9222888 |
|
Current U.S.
Class: |
600/547;
600/484 |
Current CPC
Class: |
A61B
5/0536 (20130101) |
Current International
Class: |
A61B
5/053 (20060101); A61B 005/02 () |
Field of
Search: |
;128/693,723,734,898,897 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
|
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|
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2160323 |
|
Dec 1985 |
|
GB |
|
9119454 |
|
Dec 1991 |
|
WO |
|
Other References
Kanai et al., "Electrical measurement of fluid distribution in legs
and arms", Medical Process through Technology, 12:159-170 (1987).
.
Poc. 11th Ann. Conf. of the IEEE Eing. In Med. and Biol. Society.
vol. 11, 12 Nov. 1989, Seattle, WA (US) pp. 476-477, H.Griffiths et
al. "Dual-Frequency Eit in vitro and in Vivo". .
Proc. Ninth Ann.Conf. of the IEEE Eng. In Med. and Biol Soc., vol.
9, 16 Nov. 1987, Boston, MA (US) pp. 1416-1417 Zhili Huang et al.
Bioimpedance Measurement: Theory, Experiment and Application. .
Clinical Physics and Physiological Measurement, Supplement A, vol.
13, 1992, pp. 67-72, P.M.Record et al "Multifrequency Electrical
Impedance tomography" cited in the Appln., Abstract and Section 6.
Demonstration. .
IEEE Eng. In Medicine and Biology, vol. 8, No. 1, Mar. 1989, New
York (US) pp. 11-15, XP2285 L.E. Baker, "Principles of Impedence
Technique" Sections Reactive Component in bioimpedance and
Impedance Imaging..
|
Primary Examiner: Bahr; Jennifer
Assistant Examiner: Gilbert; Samuel
Attorney, Agent or Firm: Cushman, Darby & Cushman IP
Group of Pillsbury, Madison & Sutro LLP
Claims
We claim:
1. A method of electrical impedance investigation of a body
exhibiting a change in internal state, said method comprising:
applying to the body interrogatory electrical signals at different
frequencies;
obtaining first signals representing first electrical impedance
measurements at the different frequencies before said change in the
internal state of the body;
obtaining second signals representing second electrical impedance
measurements at the different frequencies after said change in the
internal state of the body, wherein the first and second obtained
signals are associated with a variation in two variables, said
variables being (i) frequency and (ii) internal state of the
body;
determining a change in said impedance measurements over a
variation of one of said two variables;
determining a normalized change measurement from said change in
said impedance measurements, said change in said impedance
measurements being normalized with respect to a chosen reference
impedance measurement;
determining a response, over a variation of said other of said
variables, to said normalized change measurement; and
selectively determining characteristics of a part of said body from
said response.
2. A method according to claim 1, wherein the change in the
internal state of the body involves a change in geometry of at
least part of said body.
3. A method according to claim 1 or claim 2, wherein said first
electrical impedance measurements and said second electrical
impedance measurements correspond to selected different points in a
cyclic change in the internal state of the body.
4. A method according to claim 3, wherein the respective
measurements are made at points selected to correspond
substantially to peak and trough points in the cyclic change.
5. A method according to claim 1, wherein the obtained signals
represent only a real part of the electrical impedance
measurements.
6. A method according to claim 1, wherein the electrical impedance
measurements are made at frequencies in the kHz range.
7. A method according to claim 6, wherein there is at least one
order of numerical difference between the lowest and highest
frequency.
8. A method according to any preceding claim 1, wherein the body is
a human or animal body of a subject.
9. A method according to claim 8, wherein the body part is an organ
or organs of the body.
10. A method according to claim 8 or claim 9, wherein the body has
a trunk and the interrogatory electrical signals are applied to
said trunk, said part of the body being the heart or at least one
lung of the body.
11. A method according to claim 10, wherein said first electrical
impedance measurements are made when the subject's breath is held
after inspiration and said second electrical impedance measurements
are made when the subject's breath is no longer held.
12. A method according to claim 9, wherein the respective signals
are obtained by taking measurements in synchronization with a
cyclic variation in the state of said part of the body.
13. A method according to claim 1, including using electrical
impedance tomography (EIT) techniques to generate images of said
part of the body by use of signals representing said
characteristics of said part of the body.
14. A method of electrical impedance investigation of a body
exhibiting a change in internal state, said method comprising:
applying to the body interrogatory electrical signals at different
frequencies;
obtaining first signals representing first electrical impedance
measurements at the different frequencies before said change in the
internal state of the body;
obtaining second signals representing second electrical impedance
measurements at the different frequencies after said change in the
internal state of the body;
selectively determining characteristics of a part of said body from
a response to a change in frequency of a differential variation
between the first and second signals;
wherein the body is a human or animal body of a subject;
wherein said electrical signals are applied to the trunk of the
body and the part of said body is the heart or at least one lung of
the body; and
wherein said first electrical impedance measurements are made when
the subject's breath is held after inspiration and said second
electrical impedance measurements are made when the subject's
breath is no longer held.
15. A method of electrical impedance investigation of a body
exhibiting a change in internal state, said method comprising:
applying to the body interrogatory electrical signals at different
frequencies;
obtaining first signals representing first electrical impedance
measurements at the different frequencies before said change in the
internal state of the body;
obtaining second signals representing second electrical impedance
measurements at the different frequencies after said change in the
internal state of the body;
selectively determining characteristics of a part of said body from
a response to a change in frequency of a differential variation
between the first and second signals; and
wherein electrical impedance techniques are used to generate images
of said part of said body by use of signals representing said
characteristics of said part of said body.
Description
BACKGROUND OF THE INVENTION
Field of the invention
This invention concerns investigation of a body. More specifically
it is related to tomography and particularly to electrical
impedance tomography or so-called EIT.
DESCRIPTION OF THE RELATED ART
EIT as currently developed (see B. H. Brown and D. C. Barber,
Electrical ImpedanceTomography, Clin. Phys. and Physiol Meas. 13,
Suppl. A., pp.207, 1992) uses an array of electrodes placed around
a body, which is normally that of a human patient, to produce an
image of changes in tissue resistivity or impedance. It has been
shown that both cardiac and respiratory related changes can be
imaged. The respiratory related changes arise mainly from the lungs
but the cardiac related changes arise from the heart, the lungs and
the major blood vessels. The changes from the heart arise from
large changes in blood volume. The changes from the blood vessels
are due mainly to changes in cross sectional area, and hence blood
volume, as the pulse pressure changes. Changes from the lungs are
also associated with blood volume changes as the pressure on
systole increases the blood content of the pulmonary tree. EIT
images of cardiac and respiratory related changes have been made at
a single frequency within the range 20-50 kHz. International
application W091/19454 describes a real-time EIT system applicable
to the investigation of dynamic systems, such as the observation of
blood flow in the human body during the cardiac cycle.
Several EIT research groups are now considering making images from
measurements made over a range of frequencies, with a view to
reducing the effects of body geometry on the images and also in the
hope that tissue can be characterized in terms of how its impedance
changes with frequency. Such work is described in, among other
papers, P. M. Record, R. Gadd and F. Vinther, Multi-frequency EIT,
Clin. Phys. and Physiol. Meas. 13, Suppl. A, pp.67-72(1992). H.
Griffiths and Z. Zhang, Dual-Frequency Electrical Impedance
Tomography in Vitro and in Vivo, Proc. 11th Ann. Conf. of the IEEE
Eng. in Med. and Biol. Society, Vol. 11, 12 November 1989, pp.
476-477, similarly describes imaging using dual-frequency EIT.
SUMMARY OF THE INVENTION
The present invention derives from investigations of electrical
impedance measurements made on the human trunk over a wide
frequency range which have led to results of an unexpected nature.
This work was directed towards the study of changes of impedance
occurring over a change in the internal state of a body measured at
different electrical signal frequencies.
According to the invention there is provided a method of
investigation of a body comprising:
applying interrogatory electrical signals at different frequencies
to the body;
obtaining first signals representing first electrical impedance
measurements at the different frequencies;
obtaining second signals representing subsequent second electrical
impedance measurements at the different frequencies after a change
in the internal state of the body; and
selectively determining characteristics of part of said body in
response to the first and second signals at the different
frequencies.
The work referred to above and described in greater detail below
has led to the surprising finding, hitherto not appreciated, that
there can be a significant difference in the way in which impedance
changes in a dynamic system vary with frequency, depending on which
part of the system the impedance or impedance change is associated
with.
More specifically, in the case of in vivo investigations of the
human or animal body and considering impedances in the trunk of the
body, as the frequency increases, the impedance change associated
with the cardiac cycle falls considerably more rapidly than that
associated with the respiratory cycle. This phenomenon has not
anticipated by any studies hitherto made. The temporal change in
impedance associated with different dynamic features of the body is
found to be a function of frequency, the function depending on
which dynamic feature the impedance change is associated with.
In most cases, the change in an internal state of the body is a
change in the geometry of at least a part of the body. For example,
in the human body, such changes may be changes in the cross
sectional area of blood vessels due to a pulsatile blood flow.
Alternatively, such changes may be changes in the air volume, and
hence, the size of the lungs, or changes in blood volume in the
heart. Non medical or veterinary applications of the invention are
also envisioned.
Preferably the electrical impedance measurements are made at
selected different points in a cyclic change in the internal state
of the body. Preferably, these points are selected to correspond
substantially to the peak and trough of the cyclic fluctuation in
impedance associated with the cyclic change.
In the case of the human body, the respective signals can be
obtained by taking measurements in synchronization with a cyclic
variation in the state of a particular body part. For example,
cardiosynchronous signals can be related to the heart, the
measurements being synchronized with a point in the subject's ECG
wave, while measurements synchronized with a subject's inspiration
and expiration can be related to the lungs.
Preferably, the signal representations obtained represent only the
real part of the impedance measurements. In many cases, it is
thought that the capacitive components of impedance measurements
can be prone to errors and are therefore unreliable.
In a developed form of the invention, the method involves
generating a tomographic image of the body part using signals
representing the characteristics of the body part.
The observed phenomenon can, for example, be used in generating
tomographic images of a cross section of a human body. By taking
advantage of the different frequency behavior of organs associated
with temporal changes, organ resolution and tissue differentiation
can be improved.
The electrical signals are suitably in the kHz frequency range with
there preferably being at least one order of numerical difference
between the lowest and highest frequencies. It is considerably
easier to work in the kHz range than in the MHz range.
BRIEF DESCRIPTION OF THE DRAWINGS
Further detail of the invention can be appreciated from the
following description with reference to the accompanying drawings,
in which:
FIG. 1 diagramatically illustrates the tests carried out on a group
of human subjects to obtain frequency-based impedance
measurements;
FIGS. 2a and 2b show the results of two of the tests carried out on
a subject at two different frequencies;
FIG. 3 represents a simple form of electrical equivalent circuit
for tissue;
FIG. 4 shows the results of a different way of modelling of tissue
impedance characteristics;
FIG. 5 shows, as a function of frequency, the resistivity change
between inspiration and expiration of a human subject associated
with different regions of interest within a body; and
FIG. 6 shows tomographic images of the resistivity change between
inspiration and expiration at different frequencies.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The investigations leading to the present invention will now be
described in more detail.
Tetrapolar impedance measurements were made as shown in FIG. 1 by
applying a sinusoidally varying current of constant 1 mA peak to
peak amplitude between the left upper arm 1 and left calf 2 of a
human subject and measuring the potential resulting between the
right upper arm 3 and right calf 4. The measurements obtained
should principally relate to the impedance of the trunk 5. The
electrodes used were Ag/AgCl types (Conmed 140-2545 [Trademark])
and the electrode impedances measured were typically <500 ohms
at 9.6 kHz.
The current waveform was successively doubled in frequency from 9.6
kHz to 614.4 kHz and each step lasted for 3.33 ms. This allowed a
complete set of measurements at 7 frequencies with a 3.33 ms gap
between sets to be made in 26.7 ms, thereby giving 37.5 data sets
per second. The potentials recorded were first amplified and then
de-modulated using a high frequency multiplier to extract the
in-phase component of the signal, in other words, the real part of
the complex impedance. AC coupling used within the amplifier
allowed the signal to settle to better than 0.1% within the 3.3 ms
periods. The resulting signal was then low pass filtered (4 pole
with 3 dB at 25 Hz) before being digitized at 50 or 200 samples per
second to 12 bit resolution and passed to a computer. The A-D
interface was a DAS-8PGA8 (Keithley) and the computer a Research
Machines 386/20. Data was collected using commercial software
(Asyst, EasyestLX, Keithley). The system was calibrated using
resistances up to 20 ohms and parallel resistor-capacitor
combinations (300 ohms and 10 nF) to represent the electrodes. The
measurement accuracy was within 3% over the 7 frequencies.
Twelve normal subjects with no known respiratory or cardiac
abnormalities were used for the measurements (Average age 37 years;
range 23 years to 51 years; 9 male and 3 females). Although not
shown in FIG. 1, the subjects were seated on an insulating surface
during the measurements and placed their hands on a wooden bench.
Care was taken to see that the knees did not touch during the
measurements as this would present an uncontrolled path for current
flow.
Two recordings, or measurement sets, were made from each subject,
the first recording was 10 seconds duration and the second
recording was 40 seconds. During the first recording the subject
was asked to inspire total lung capacity and to hold his or her
breath for the 10 second recording. During the second recording,
each subject again held his or her breath for 10 seconds but was
then told to expire and breath normally for an additional 30
seconds. Each recording was collected as 2048 data points.
The first recording was used to measure the real part of the
absolute impedance at the seven frequencies and also the amplitude
of the cardiac related changes. The second was used to record the
amplitude of the impedance change on expiration following the
breath hold. Because of the high frame rate of 37.5 s.sup.-1 the
measurements at the 7 frequencies were in effect made
simultaneously. The measurements of the amplitudes of the cardiac
and respiratory related components were made by printing the
waveforms and then manually measuring the peak to peak amplitude of
the signals. In every case, the waveforms were digitally low pass
filtered at 10 Hz in order to reduce noise. It was found that no
attenuation of the cardiac related changes occurred as a result of
the filtering.
A trace of the impedance measured at 9.6 kHz from one of the 40
second recordings is shown in FIG. 2a. The cardiac related changes
are clearly visible during the first 12 seconds when the breath is
held. On expiration the impedance falls and the changes during
tidal breathing can be seen. The cardiac changes show a rapid
decrease in impedance during systole and then a slower increase
during diastole. The ECG was recorded in one subject and confirmed
that the rapid decrease in impedance did correspond to the start of
cardiac systole.
FIG. 2b shows the measurements made at 307 kHz in the same
recording as that shown in FIG. 2a. It can be clearly seen that the
relative amplitude of the cardiac and respiratory related changes
are different at the respective frequencies, the cardiac related
changes being smaller in the recording made at 307 kHz.
The group mean values of impedances measured at the 7 frequencies
and the amplitudes of the respiratory and cardiac related changes
are given in Tables 1 and 2 presented below. The mean impedance and
the cardiac related changes were measured during the 10 second
breath hold at total lung capacity. The respiratory component was
measured from the second recording as the change from total lung
capacity to normal tidal breathing. In all cases, it is the real
part of the impedance which is shown so that impedances are given
as ohms in the case of the respiratory changes and milli-ohms in
the case of the cardiac related changes. The recording illustrated
in FIGS. 2a and 2b show mean impedances at total lung capacity of
about 21.2 ohms at 9.6 kHz and 14.2 ohms at 307 kHz. These figures
compare with the group means of 24.56 ohms at 9.6 kHz and 17.81
ohms at 307 kHz. Standard deviations are given for all the
measurements.
These standard deviations are quite large because they depend upon
the shape and size of the subject as well as on the internal
resistivities. In Table 2 normalized results are presented such
that 100% represents the measurement at 9.6 kHz. The standard
deviations given here are significantly less than in Table 1
because they show only the change in impedance with frequency in
each individual.
______________________________________ Impedance Respiratory
Cardiac component Frequency (.OMEGA.) component (.OMEGA.) (.OMEGA.
.times. 10.sup.-3) (kHz) Z .delta.Z.sub.r .delta.Z.sub.c
______________________________________ 9.6 24.56 .+-. 4.09 1.68
.+-. 0.43 98.4 .+-. 33.0 19.2 23.93 .+-. 4.10 1.66 .+-. 0.43 89.1
.+-. 32.5 38.4 22.64 .+-. 4.08 1.62 .+-. 0.42 79.4 .+-. 32.7 76.8
21.04 .+-. 3.96 1.55 .+-. 0.41 63.5 .+-. 26.2 153.6 19.35 .+-. 3.78
1.47 .+-. 0.39 54.1 .+-. 22.5 307.2 17.81 .+-. 3.50 1.37 .+-. 0.36
40.4 .+-. 17.9 614.4 15.70 .+-. 2.80 1.10 .+-. 0.30 28.2 .+-. 12.4
______________________________________
Table 1, presented immediately above, shows mean data for the 12
subjects. The respiratory component (.delta.Z.sub.r) is the
respiratory related impedance change, i.e. the impedance change
between total lung capacity (breath held) and normal tidal
breathing. The cardiac component (.delta.Z.sub.c) is the cardiac
related impedance change measured from the peak to peak amplitude
of the fluctuating impedance signal at total lung capacity.
______________________________________ Impedance Respiratory
Cardiac component Frequency (.OMEGA.) component (.OMEGA.) (.OMEGA.
.times. 10.sup.-3) (kHz) Z .delta.Z.sub.r .delta.Z.sub.c
______________________________________ 9.6 100 100 100 19.2 97.4
.+-. 1.1 98.9 .+-. 1.2 89.9 .+-. 6.2 38.4 92.0 .+-. 1.4 96.5 .+-.
2.3 78.7 .+-. 8.9 76.8 85.4 .+-. 2.0 92.1 .+-. 3.4 63.1 .+-. 9.1
153.6 78.4 .+-. 2.5 87.5 .+-. 3.8 53.7 .+-. 8.6 307.2 72.2 .+-. 2.4
80.1 .+-. 5.2 40.7 .+-. 10.5 614.4 64.0 .+-. 2.1 65.4 .+-. 3.4 28.2
.+-. 6.5 ______________________________________
Table 2, presented immediately above, shows mean data for the 12
subjects. The values have been normalized to the measurements made
at 9.6 kHz before taking the mean for the group.
In all twelve cases, the amplitude of the cardiac related impedance
changes (.delta.Z.sub.c) decreased more rapidly with increasing
frequency than did the respiratory related impedance change
(.delta.Z.sub.r). The cardiac change fell from 100% to 28.2% (range
18.6-39.4%). The respiratory change fell from 100% to 65.4% (range
59.8-71.1%). A decrease of impedance with frequency has typically
been found in biological tissue, but the difference in the case of
the cardiac and respiratory components was unexpected. Subsequent
studies by the inventors have confirmed these findings, and tests
carried out with the subject following different breathing patterns
and using various different configurations of electrodes have also
produced results consistent with those described above. Because the
relaxation frequency for blood has been quoted as typically 1-3 Mhz
it was expected that the cardiac related changes would not fall
significantly in amplitude at frequencies up to 600 kHz. Some
modelling was carried out in order to investigate this observation
of a relatively rapid fall in the amplitude of the cardiac related
changes.
Many studies have used a simple R C combination as shown in FIG. 3
to model tissue impedance. In this case R can be loosely related to
extracellular conduction and S to intracellular conduction across
the membrane capacitances represented by C. However, a close fit
cannot be obtained to in vivo data because there is a dispersion of
time constants in tissue; In other words, there is a range of time
constants present in tissue.
Some studies have used susceptance vs. conductance plots (H. Kanai,
M. Haeno and K. Sakamoto, Electrical measurement of fluid
distribution in legs and arms, Medical Progress through Technology,
12,159-170, 1987) based upon the work of Cole and Cole (K. S. Cole
and R. H. Cole, Dispersion and Absorption in Dielectrics, Journal
of Chemical Physics, 9, 341-351, 1941) to obtain the loci of tissue
equivalent circuits. These require measurements to be made over
many frequencies and require both real and imaginary components to
be available. In the study described above, measurements at only
seven frequencies were made and only the real part was recorded. It
is possible to record the out-of-phase component, but it is thought
that cable capacitances and body capacitance to ground make in vivo
recording of the out-of-phase component unreliable. Because of this
and the fact that measurements were only made at seven frequencies,
the results have been modelled as follows.
A general impedance which establishes a circle in the complex
impedance plane can be written:
where R.sub..infin. is the very high frequency impedance, R.sub.O
the low frequency impedance, f the frequency, f.sub.r the
relaxation frequency for the tissue and .alpha. is the constant
which characterizes the Cole-Cole distribution function. Extracting
the real part of equation (1) gives: ##EQU1##
To be consistent with the notation of FIG. 3, R.dbd.R.sub.O and
R.sub..infin. .dbd.RS/(R.dbd.S).
This has been used to perform a least square fit to the data of
Table 1. Parameters R,S,f.sub.r and .alpha. were thus obtained for
the total impedance, the respiratory component and the cardiac
component. The results of this modelling are illustrated in FIG. 4,
which shows both the measured values of the different impedance
components and the equivalent values from the model. The curves are
measured absolute impedance 20, measured cardiac component 21,
measured respiratory component 22, modelled absolute impedance 23,
modelled cardiac component 24 and modelled respiratory component
25, all shown as percentages of that measurement at 9.6 kHz.
The modelling shows that the measured data can be modelled very
well to equation (2). The following parameters were obtained.
______________________________________ Total Impedance: R =
26.13.OMEGA. S = 23.38.OMEGA. f.sub.r = 158 kHz .alpha. = 0.39
Respiratory related component: R = 1.68.OMEGA. S = 1.44.OMEGA.
f.sub.r = 421.5 kHz .alpha. = 0.21 Cardiac related component: R =
118.7.OMEGA. S = 7.26.OMEGA. f.sub.r = 86.8 kHz .alpha. = 0.43
______________________________________
It can be argued that it is more logical to see the cardiac and
respiratory related components as a perturbation of the base
impedance. If the modelling is done in this way, then we can find
the equivalent changes in R and S which explain the observations.
The results of this are as follows.
______________________________________ Respiratory related
component: change in R is 1.45.OMEGA. change in S is 3.83.OMEGA.
Cardiac related component: change in R is 0.104.OMEGA. change in S
is 0.136.OMEGA. ______________________________________
As can be seen from the results above the measurements of trunk
impedance fall with a frequency which can be modelled very well
using a Cole-Cole equation. The fall in impedance in an individual
is very consistent. However, the differences in absolute impedances
between individuals are quite large and probably largely reflect
differences in body shape and size. If a simplified model were
assumed for the trunk then it would be possible to express the
impedance in terms of tissue volume. However, such a simplified
model would require so many assumptions that its utility would be
questionable.
The results show a significant difference in the way in which the
cardiac and respiratory related impedance changes vary with
frequency. If a multi-frequency electrical impedance tomographic
(EIT) system is used to calculate the impedance spectrum for each
pixel, then it is possible to identify cardiac and respiratory
signals on the basis of their different frequency behavior. Single
frequency EIT imaging has demonstrated images of impedance changes
from both the heart and lungs. However, the spatial resolution is
poor and in many cases it is not possible to differentiate tissues.
The in vivo images published have used a 2-D solution to fit a 3-D
problem and the spatial resolution is at best 10% of the imaged
diameter. However, based on the present invention, by making
multi-frequency measurements, it is possible to identify tissues on
the basis of the impedance spectrum and the spectrum of the changes
in impedance.
It was expected that the tissue impedance would fall with frequency
and that the changes during lung ventilation would fall in a
similar manner. However, if the cardiac related changes arise from
changes in blood distribution, then perhaps only small changes with
frequency would be expected. Blood has no alpha dispersion but a
beta dispersion is observed. A relaxation frequency (i.e., the 3 dB
frequency corresponding to the mean tissue time constant), for
blood of 3 MHz has been suggested (Kanai), and other studies have
found values about 1-2 MHz. If these values are correct, then over
the frequency range of the measurements of the tests described
above (9.6 to 614 kHz), relatively small changes should be seen.
The modelling of the cardiac related changes showed a relaxation
frequency of 86.8 kHz, which is not consistent with Kanai's figures
for blood.
One possible explanation of the observations of the tests described
above is that the origin of the cardiac related impedance changes
could be the pulsatile blood volume changes in the upper part of
the pulmonary tree. These could be shunted by the non-pulsatile
lung tissue that has a decreasing impedance at high frequencies,
and thus, decreases the relative magnitude of the cardiac related
impedance changes. It has been shown that cardiac related impedance
changes can be recorded from the area of the lungs, and that these
might arise from changes in blood volume in the lungs. Most of
these changes are likely to occur in the larger vessels where the
pressure waveform is pulsatile, and hence, changes in vessel
cross-section occur. Two reasons can be found to identify the lungs
as the major possible origin of the cardiac related changes. First,
the impedance falls during systole whereas one would expect the
impedance of the heart to rise, and second, for a large undispersed
volume of blood, such as is found in the heart and major vessels,
one might expect to see the high relaxation frequency found from
measurements on whole blood. It is, however, not possible to make a
straightforward comparison between the frequency dispersion of
blood and that of the whole trunk.
Further investigations have been carried out into the frequency
response of the impedance changes associated with different regions
of a tomographic image of a cross section of a human trunk. FIG. 5
graphically shows the results of these investigations, the
horizontal axis representing frequency and the vertical axis
representing the ratio, expressed as a percentage, of the change in
resistivity between inspiration and expiration referenced to
resistivity on expiration, (Z insp-Z exp)/Z exp.
The subject was told to breath in fully and hold his breath, and
then to expire fully and again hold his breath. The four curves 30,
31, 32 and 33 show the frequency response of different areas of
clinical interest, and respectively represent the right lung
response, the left lung response, the spinal column response, and
the response of the central region of the image, which is thought
to represent the cardiovascular system.
The curves of FIG. 5 clearly show how, by examining the frequency
response of the dynamic impedance changes, selective determination
of different parts of the tomographic image is possible. The
gradients of the curves, especially in the 0-100 kHz frequency
range, show a clear distinction for different anatomical regions of
interest. As expected, the response associated with the spinal
column is virtually a flat line, as the spinal column exhibits
minimal change in state over the respiratory cycle. Any change in
this value is probably due to some contamination by lung data in
the selected region.
FIG. 6 shows a set of EIT images illustrating the use of the
above-mentioned effect. The eight images were produced using a
standard EIT technique, the impedance measurements being made
effectively simultaneously at eight different electrical signal
frequencies, from 9.6 kHz to 1228.8 kHz, successive images
representing a doubling of the frequency. The pixel values in the
images represent the values of (Z insp-Z exp)/Zexp over the
respiratory cycle, i.e., the resistivity change referenced to
resistivity at expiration. The images are normalized to the maximum
change in the image set. The progressive changes in the images as
the frequency is increased illustrate the use of the technique of
the invention, such changes being in no way predicted prior to the
invention.
The values used to produce the curves at FIG. 5 are taken from
these images by selectively localizing regions of interest on an
image and determining resistivity values for those regions. More
particularly, for each lung region, the image data were examined to
find the peak change in resistivity associated with that lung over
the respiratory cycle at 9.6 kHz. A region was then determined
including pixels displaying a change in resistivity over the cycle
of 50% and above of this peak change. This region was assumed to
represent the area of that lung. The values used for the curves of
FIG. 5 represent mean values over the relevant region. A similar
approach was used to fix a central region of the image, and
therefore, to produce curve 33 in FIG. 5. For the spinal region,
this was fixed by eye from dual frequency static imaging, and again
the values used to construct curve 32 in FIG. 5 represent mean
values over that region.
The invention has been described and illustrated with reference to
a cardiac and respiratory related impedance response. However, it
is to be understood that it may also be applied to investigate
different parts of the human or animal body where a change in
internal state occurs. For example, the technique may find
application in investigating the movement of food or fluid through
a subject's oesophagus or the movements of a subject's gastric
contents. The movement of the contents or peristaltic effects will
provide the change in internal state. The technique might also find
application outside medical or veterinary areas.
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